GM 350 Firing Order: 1-8-4-3-6-5-7-2 – Diagram, Animation, & Pro Tech Details
🔍 Why 1-8-4-3-6-5-7-2? — Engineering Deep Dive
The GM 350 firing order was selected to achieve crankshaft counterweight balance and even intake manifold pressure waves. This sequence (known as the “18436572” firing order) spreads combustion loads across the journal throws, reducing main bearing stress. Additionally, it pairs cylinders that are 90° apart in crank rotation, giving the classic small-block V8 its distinctive lumpy idle when equipped with performance cams.
Types of firing orders in V8 engines: While Ford uses 1-5-4-2-6-3-7-8 (351W) and LS engines use 1-8-7-2-6-5-4-3, the GM SBC 350 retains the traditional 18436572. This cross-plane crank design ensures that each bank fires alternately, reducing exhaust backpressure pulses.
✔ Smooth idle & low-end torque
✔ Excellent harmonic damping
✔ Wide powerband (1,500–5,500 RPM)
✔ Easy to diagnose with wire trace
✘ Severe engine misfire
✘ Backfire through intake
✘ Catalytic converter damage
✘ Bent valves (if severe)
• 1967–2003 Chevy C/K trucks
• Camaro, Corvette, Impala SS
• Marine 5.7L Mercruiser
• Crate engines & hot rods
📋 How To: Verify & Set Firing Order on GM 350 (Step-by-Step)
How to correctly set the firing order: Follow this professional mechanic sequence:
- Locate #1 TDC compression stroke – Remove #1 plug, place finger over hole, crank until air pushes out. Align timing mark (0°).
- Install distributor – Rotor should point toward #1 terminal on cap (usually marked or near #1 cylinder).
- Connect spark plug wires in clockwise direction following 1-8-4-3-6-5-7-2 around the distributor cap.
- Double-check cylinder numbering: driver side 1-3-5-7 front to rear, passenger side 2-4-6-8.
- Final test – Start engine, use timing light to set base timing (typically 8-12° BTDC for stock 350).
📊 Distributor Cap Wiring Reference Table (GM 350 HEI)
| Dist. Tower Position (Clockwise from #1) | Cylinder Number | Firing Order Step |
|---|---|---|
| #1 (reference / marked) | 1 | 1st |
| Next clockwise | 8 | 2nd |
| 2nd clockwise | 4 | 3rd |
| 3rd clockwise | 3 | 4th |
| 4th clockwise | 6 | 5th |
| 5th clockwise | 5 | 6th |
| 6th clockwise | 7 | 7th |
| 7th clockwise | 2 | 8th (completes cycle) |
🛡️ Is It Safe to Modify the GM 350 Firing Order?
Is it safe? Absolutely not. The engine’s camshaft timing, crankshaft counterweights, and even the ECU (on later models) are hard-tuned to the 1-8-4-3-6-5-7-2 sequence. Changing wires arbitrarily results in violent backfires, engine stalling, unburned fuel in exhaust and can damage pistons. Only performance camshafts designed for a “4/7 swap” (different firing order) are acceptable, but they require specific matching components.
💡 Advanced Insights: Firing Order & Engine Balance, Exhaust Note
The GM 350 firing order creates a firing interval of 90° crankshaft degrees with alternating banks: Left Bank (1,7) fires, then Right Bank (8,4,6,2) and so on. This pattern gives the small-block a characteristic “burble” and contributes to excellent scavenging when using dual exhaust. Moreover, this order reduces the second-order vibrations, resulting in a smoother V8 compared to flat-plane cranks.
🖥️ Common Symptoms of Incorrect GM 350 Firing Order
- Rough idle or no start – engine cranks but won’t fire or shakes violently.
- Backfiring through throttle body / carburetor – indicates wire order swapped between adjacent cylinders.
- Loss of power & excessive fuel smell – unburned fuel due to wrong ignition timing.
- Check engine light (OBDI/OBDII) – random misfire codes P0300–P0308.
📈 Comparative Analysis: GM 350 vs Other V8 Firing Orders
| Engine | Firing Order | Distributor Rotation | Notes |
|---|---|---|---|
| GM 350 SBC | 1-8-4-3-6-5-7-2 | Clockwise | Classic small-block |
| Ford 351W | 1-5-4-2-6-3-7-8 | Counter-clockwise | Different vibration pattern |
| LS1 / LS6 (Gen III) | 1-8-7-2-6-5-4-3 | Clockwise | Improved main bearing load |
| Chrysler LA 360 | 1-8-4-3-6-5-7-2 | Clockwise | Similar to GM 350 |
📝 Troubleshooting & Diagnostic Guide
How to check without starting: Remove distributor cap, crank engine to #1 TDC (compression). Confirm rotor points to #1 post. Then, verify each wire follows the 1-8-4-3-6-5-7-2 order using an ohmmeter or visual trace. Use our live animation above to mentally match the firing sequence while rotating the engine manually.
Tools needed: Timing light, spark tester, firing order diagram (this page), 5/8″ wrench for crankshaft pulley, and a sharpie to label wires.
